Organic photovoltaics have captured much attention because they are lightweight and mechanically flexible, allowing them to occupy new niches for solar power like integration with clothing and building materials. They are also manufactured using a low-cost, roll-to-roll process, which is key if they are to successfully compete with silicon-based PV. However, one limitation is the current requirement for relatively costly fullerenes as the electron acceptor material. Now, H. Huang and colleagues at Northwestern University, the University of the Chinese Academy of Sciences, the University of Málaga, and Polyera Corporation have synthesized and characterized a new series of alkoxy-functionalized, π-conjugated polymers that show significant promise for use as electron acceptors, enabling an all-polymer solar cell with power conversion efficiency of 1.70%. They recently reported their results in Advanced Functional Materials (DOI: 10.1002/adfm.201303219).
Typical organic photovoltaic cells (OPVs) are based on an active organic layer sandwiched between electrodes (usually indium tin oxide and aluminum). Since this system can be built on a plastic substrate, the resulting devices are lightweight and mechanically flexible, opening a wide range of potential applications. Of particular interest are bulk heterojunction (BHJ) architectures, in which the active layer is a blend of a p-type polymer electron donor and a small-molecule fullerene electron acceptor. These materials are co-dissolved in a solvent and then printed, enabling low-cost, roll-to-roll manufacturing. However, OPV efficiencies remain below that of silicon PV. Replacing the fullerenes with a polymer—thus creating all-polymer solar cells—would have several advantages, including cost reductions, increased spectral sensitivity in the red and infrared range, and enhanced polymer mechanical properties.
In pursuit of this goal, the Northwestern team had previously developed a new bisthienyl-vinylene polymer building block; when this was copolymerized with (1) benzodithiophene (BDT) as an electron donor, and (2) napthalenediimide (NDI) as an in-chain electron acceptor, the resulting p- and n-type polymers show high electron mobility and excellent ambient stability.
In the current work, the researchers built on these prior results to investigate variations of the polymer architecture, including the effects of different alkyl and alkoxy side chains, in order to develop a new series of polymers for optoelectronic applications. Tobin Marks, one of the co-authors, said, “Alkoxy groups have a wonderful ability to weakly bond with nearby sulfur groups; the interaction is sufficiently weak to give good processability in solution, but in films, is strong enough to lock the polymer backbone into place and planarize the structure in the active layer.”
After synthesizing the polymers through Stille polycondensation, the researchers found them to be thermally stable up to 280°C and to have optical absorption spectra with peaks in regions consistent with DFT calculations. One n-type NDI-containing copolymer with branched alkyl side chains was envisioned to be particularly suitable as an OPV acceptor material; encouragingly, when it was blended with the polymer donor PTB7, the system displayed almost completely quenched photoluminescence, a signature of good charge transfer from donor to acceptor and thus good photovoltaic response. The researchers tested solar cells based on this blend and found them to have a power conversion efficiency of 1.70%, the highest reported to date with NDI as an electron acceptor. To better understand the potential of the polymer series, a thin-film transistor based on this same blend was also constructed and was found to have a high electron (hole) mobility of 5.1 (3.9) × 10–3 cm2/V/s.
One important remaining issue in all-polymer OPV device performance is the optimal morphology of the organic thin film. This can be difficult to control with polymer–polymer blends, which tend to phase separate. As Rafael Verduzco of Rice University notes, the ideal microstructure is still unclear: “You probably want well-defined domains of about 10 nanometers, but recent polymers like PTB7 are challenging this thinking.” Verduzco’s group is investigating an approach in which donor and acceptor monomers are block copolymerized, and self-assemble into block lamellar micro-domains approximately 9 nm in size. Using this technique, they have demonstrated an all-polymer OPV device with an efficiency of 3.1%.
Organic electronics are increasingly entering commercial use, and the results of this research are obviously of great interest to Polyera, an Illinois-based company whose markets include wearable electronics, flexible mobile displays, and disposable electronics for supply-chain tracking (one co-author, Antonio Facchetti, is a co-founder and Chief Scientific Officer; Tobin Marks is the other co-founder). Massachusetts-based Konarka was commercializing fullerene-based OPV cells until its bankruptcy in 2012, a fact that has certainly been noted by other aspiring organic electronics companies.
While organic electronics are still not competitive with silicon-based systems for most commercial applications, it is clear that they offer some unique properties and are making good progress in closing the gap. It may not be long until OPV throws the solar market a curve—literally.
